This invention relates to a toner for developing electrostatic latent images in electrophotography, electrostatic recording, electrostatic printing or the like, in more particularly, excellent in positive chargeability.
Electrostatic latent images are developed generally as follows; First, electrostatic latent images, which are positive electric charges or negative electric charges formed on a photosensitive member by various method, attract electrostatically a toner which is charged oppositely to electrostatic latent images to form toner images, and then the toner images are transferred electrostatically onto copying paper to fix the toner. Therefore, a toner should be charged to an adequate level to obtain clear copied images without fogs and the like. Further, the charge level should not change with time and environmental conditions. It is desired that the decrease of charge amount or the toner aggregation is not caused by, for example, humid changes, because the decrease of charge amount causes toner flying, resulting in fogs on a copying ground or the dirt inside a copying machine.
In order to overcome problems above mentioned, a charge controlling agent is generally added to a toner. In a color copy process which has become popular recently, white or light yellow charge controlling agents excellent in color-reproducibility are required.
With respect to negatively chargeable charge controlling agents, there are many kinds of white or light yellow charge controlling agents available in the market. They are highly effective for charge control and have not particular limitation to practical use. But, there are few kinds of positively chargeable charge controlling agents, in particular, which can be added to a color toner, except for colored nigrosine dyes, white quaternary ammonium salts or imidazoles (Japanese Patent Laid Open Nos. 287262/1987, 259265/1986, or 187350/1984).
A nigrosine dye is not a pure chemical compound, but a mixture of more than two kinds of chemical compounds. The precise composition of a nigrosine is unidentified. Therefore, the same degree of functions can not be always expected. A nigrosine dye works effectively to give stable electric charges to a toner when the toner is used in a copying machine at mediate or low copying speed. But, a nigrosine can not be applied similarly to a toner which is constituted of a resin of low melting point and low viscosity, and such a toner can not be used in a high speed copying machine. Further, a nigrosine dye can be hardly applied to a color toner because it is colored dye.
Quarternary ammonium salts are poor in environmental resistance of charging ability, big in particle size and have stinks. Further it does not melt at fixing temperature. Moreover, an ammonium salt is not so good in chargeability as well as a nigrosine dye.
With respect to imidazole compounds, diorganotinbisimidazole (Japanese Patent Laid-Open No. 287262/1977) represented by the general formula below; ##STR1## 2-aminobenzimidazoles (Japanese Patent Laid-Open No. 217055/1986) represented by the general formula below: ##STR2## vinyl imidazole (Japanese Patent Laid-Open No. 187350/1984) represented by the general formula below; ##STR3## are known.
A charge controlling agent which is to be disclosed in the present invention is also an imidazole compound, but completely different in the structure from those above mentioned.